To be secure, identification systems need to be robust against fraud; i.e. to ensure that one person cannot pass themselves off as another person.
The field of biometrics relates to the statistical analysis of physiological characteristics. For the purposes of identification for security or other purposes, features such as fingerprints or retinal scans can be used to uniquely identify individuals. Every person has a unique set of fingerprints, and this provides a basis for identification. An image of a fingerprint can be taken and analyzed to see if it matches a recorded sample of the user's fingerprints. This is done by analyzing a number of details, or “minutiae” of the fingerprint. The greater the number of minutiae that are analyzed, the less are the chances of incorrectly identifying an individual.
However, a biometric identification system that relies solely on mathematical analysis of simple optical images can be easily spoofed, as a copy of the pattern of a fingerprint can be easily made and presented to a reader device. Accordingly, systems have been developed to identify whether the finger to be identified is indeed a three-dimensional finger, rather than just a photocopy or the like. Such a system is disclosed in U.S. Pat. No. 6,292,576 (Brownlee). A finger to be identified is placed on a platen and is illuminated by two light sources. The first light source illuminates the finger from directly below the finger, and a positive image is obtained, and the second light source is positioned at an angle greater than the critical angle so that the beam is subject to frustrated total internal reflection (FTIR) to obtain a negative image. The images can then be added, and if a true finger is present, the two images will cancel each other out, whereas if a spoof is present, the images from the two light sources will reinforce each other.
Such methods using FTIR are well explained in U.S. Pat. No. 6,292,576 and are well known in the art. However, such methods can still be spoofed, for example by rubber models of a finger. Therefore, besides analyzing the details of a fingerprint, it is desirable for a biometric identification system to verify the presence of a live finger (or other relevant body member). International Patent Publication Number WO 01/24700 gives some examples of such techniques. In this case, the ridges of the fingerprint act as one plate of a capacitor. Properties of a live finger such as perspiration, warmth, and pressure, mean that the conductivity of the ridges will change, thus affecting the capacitance. Thus, a finger can be tested with solid state capacitive sensors to see if it has the electrostatic properties that are characteristic of a live finger.
Such methods and other examples of biometric identification are well known in the art. However, they are expensive to implement. There is a need for a live finger detection apparatus that is robust against spoofing and that can be easily manufactured, installed and used.